Influence of Fe(II) on the Formation of the Spinel Iron Oxide in Alkaline Medium

J. P. Jolivet, P. Belleville, E. Tronc and J. Livage
Chimie de la Matière Condensée (CNRS URA 1466), Université Pierre et Marie Curie, Paris 05, France

Abstract: Fe(II) and Fe(III) in various proportions were coprecipitated by NH3 at pH ≈ 11. The Fe(II)/Fe(III) ratio (x) was varied from 0.10 to 0.50. After stabilization by aging at pH ≃ 8 in anaerobic conditions, hydrous precipitates were characterized by electron microscopy, Mössbauer spectroscopy, and kinetics of dissolution in acidic medium. At any x value, all stable products exhibited the structure of (oxidized) magnetite. For x ≤ 0.30, two distinct species were coexisting: the one (“m”) was made up of ca. 4nm-sized particles with a low Fe(II) content (Fe(II)/Fe(III) ≈ 0.07), and the other (“M”) consisted of particles of larger, more or less distributed sizes, and composition Fe(II)/Fe(III) ≈ 0.33; “M” increased relative amount with increasing x. For x ≥ 0.35, “M” was the only constituent and its Fe(II)/Fe(III) ratio was equal to x. “M” is identified with (nonstoichiometric) magnetite, whereas “m” is likely to be an oxyhydroxide. Mechanisms of formation are discussed, and a phase diagram is proposed which schematizes the evolution of the coprecipitation products with x and with time. Addition of Fe(II) after the precipitation of Fe(III), instead of coprecipitation, yielded very similar results.

Key Words: Dissolution • Ferrihydrite • Maghemite • Magnetite • Mössbauer spectroscopy

Clays and Clay Minerals; October 1992 v. 40; no. 5; p. 531-539; DOI: 10.1346/CCMN.1992.0400506
© 1992, The Clay Minerals Society
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